Add ZUUL_CACHE_DIR to zuul-cloner

Set the default cache directory using the ZUUL_CACHE_DIR variable so
users don't have to pass it explicitly every time.

This change does not set up --cache-dir like the other environment
variable-based options with a --zuul prefix because I did not want to
break backwards compatibility by changing the option name.

Change-Id: I1dab9dadda3a9ccf028561b09fdd78929d1dbfe7
2 files changed